Several systems have been developed and patented over the years as part of an ongoing effort to continuously improve the safety of various lifting machinery. Embodiments herein contemplate a significant improvement over prior art both in the prevention of inadvertent nuisance tripping and the overall performance of the braking system of a multi-stage lift when actuated.
U.S. Pat. No. 4,015,686 discloses a multi-stage lift which was in commercial production for many years. In this lift, the stages include like extruded aluminum mast sections which interfit in front to back relation and are separated in the front to back direction by guide rollers. The lift stages are elevated by operation of a reeving system including a cable between a rear winch and a front carriage which passes over top and bottom pulleys in each extensible stage, a top pulley on the back stationary stage, and a pulley on the carriage. The carriage pulley and the bottom pulleys on the extensible stages are spring-biased to move downwardly in case of cable failure. Such downward movement swings a locking pawl into operating position into an adjacent recess provided by a locking channel on the adjoining stage, thus stopping further movement of the mast sections upon cable failure.
The device described in U.S. Pat. No. 4,015,686 did not consistently function as intended and was prone to failure because the locking pawl would not engage into the adjacent recess provided by the locking channel on the adjoining mast stage due to acceleration, or manufacturing alignment tolerances between the pawl and the channel, or if it did engage it would rip through the slotted channel only slowing down the inevitable collapse of the multi-stage lift system. The device, when deployed in a real life cable break situation (and when it would actually function as intended), would frequently damage the multi-stage mast assembly, which is not ideal but was generally acceptable if life, limb, or property damage was averted. In many cases a full engagement of the system would leave the multi-stage mast assembly so damaged that it would be completely unusable and unrepairable (scrap).
U.S. Pat. No. 5,645,142 discloses a multi-stage lift which has also been in commercial production for many years. In this lift, the stages comprise like extruded aluminum mast sections which interfit in front to back relation and are separated in the front to back direction by guide rollers. The lift stages are elevated by operation of a reeving system including a cable between a rear winch and a front carriage which passes over top and bottom pulleys in each extensible stage, a top pulley on the back stationary stage, and a pulley on the carriage. The braking system of this device operates by the wedging of knurled rollers between a sloped face on a first mast section and a vertical face on an adjacent mast section. The sloped face is provided by a ramp member on the first mast section. A flange projects from a bottom of the ramp toward the vertical face of the adjacent mast section. A slide rod freely extends through a vertical opening in this flange. A pair of the knurled rollers are mounted adjacent an upper end of the rod at opposite sides of the rod. The slide rod extends through a compression spring seated on the flange. Normally, this spring is engaged at the top by a stop pin on the slide rod and is compressed by the combined weight of the rod and rollers. However, if the mast stage on which the ramp member is mounted accelerates downwardly relative to the adjacent mast stage, the compression spring responsively expands and causes the wedging rollers to move up the ramp so that they are wedged between the ramp member of the first mast section and the vertical face of the adjacent mast section. In this wedging position, the knurled rollers stop further downward movement of the ramp member and associated first mast section relative to the adjacent mast stage.
The device described in U.S. Pat. No. 5,645,142 functions fairly consistently but, since it is triggered by gravity versus lack of cable tension, it is prone to inadvertent nuisance tripping. That is, the braking system sometimes engaged even when a cable failure was not present. As examples, such inadvertent trippings may occur simply by the operator lowering the multi-stage mast assembly in an accelerated fashion or, in such a similar manner, when turning the crank on the winch with uneven gyrations or oscillations which result in a situation that momentarily simulates a quick downward acceleration strong enough and for a duration long enough to simulate the free fall of a vertical mast member. These actions result in the compression spring responsively expanding and causing the wedging knurled rollers to occupy a wedging position, stopping further downward movement of the ramp member and associated mast section relative to the adjacent mast stage. The situation is compounded by the fact that these multi-stage lifts are frequently transported in pick-up trucks and other vehicles in the horizontal position, allowing the wedging rollers to move to a wedging position during transport of the lift. When the lift is placed back in the vertical position it is possible that some of the knurled rollers do not fall back into their normal operating position. The inadvertent nuisance tripping of this device causes significant customer dissatisfaction and it is not uncommon that a multi-stage lift gets stuck in the elevated position and must be carefully laid down on it back by forklifts or other devices so that a service mechanic can use a special tool to disengage the wedge roller(s) from the wedge position(s). This is an expensive situation as it involves a service call to a qualified mechanic and the act of taking a multi-stage lift that is in a vertical position 25 ft. in the air and laying it on its back in the horizontal position can be dangerous and may be very difficult to accomplish if the lift is in a tight location.